1. Field of the Invention
The present invention relates to an oxide sintered body, a target and a transparent conductive film obtained by using the same, and a transparent conductive substrate, and in more detail, the present invention relates to an oxide sintered body having zinc oxide as a main component, and still more containing magnesium, a target obtained by processing the same, a transparent conductive film having excellent chemical resistance and low resistance, obtained by a direct-current sputtering method or an ion plating method by using the same, and a transparent conductive substrate.
2. Description of the Prior Art
A transparent conductive film has high electrical conductivity and high visible light transmittance. The transparent conductive film has been utilized in an electrode or the like, for a solar cell or a liquid crystal display element, and other various light receiving elements, as well as a heat ray reflection film for an automotive window or construction use, an antistatic film, and a transparent heat generator for various anti-fogging for a refrigerator showcase and the like.
As the transparent conductive film, there has been known a thin film of tin oxide (SnO2)-type, zinc oxide (ZnO)-type, indium oxide (In2O3)-type. As the tin oxide-type, one containing antimony as a dopant (ATO), or one containing fluorine as a dopant (FTO) has been utilized. As the zinc oxide-type, one containing aluminum as a dopant (AZO), or one containing gallium as a dopant (GZO) has been utilized. The transparent conductive film most widely used industrially is the indium oxide-type. Among them, indium oxide containing tin as a dopant is called an ITO (Indium-Tin-Oxide) film, and has been utilized widely, because, in particular, a film with low resistance can be obtained easily.
The transparent conductive film with low resistance is suitably used in a display element or a touch panel or the like, such as for a solar cell, a liquid crystal, an organic electroluminescence and an inorganic electroluminescence. As a method for producing these transparent conductive films, a sputtering method or an ion plating method has been used often. In particular, a sputtering method is an effective method in film-formation of a material with low vapor pressure, or when control of precise film thickness is required, and because of very simple and easy operation thereof, it has been widely used industrially.
A sputtering method is a film-formation method using a sputtering target as a raw material of a thin film. The target is a solid containing a metal element constituting the thin film to be formed, and a sintered body such as a metal, a metal oxide, a metal nitride, a metal carbide, or, in certain cases, a single crystal is used. In this method, in general, after making a vacuuming apparatus to high vacuum once, rare gas such as argon is introduced, and under a gas pressure of about 10 Pa or lower, a substrate is set as an anode and a sputtering target is set as a cathode to generate glow discharge between them and generate argon plasma, and argon cations in the plasma are collided with the sputtering target of a cathode, and particles of the target component flicked thereby are deposited on a substrate to form a film.
In addition, a sputtering method is classified by a generation method of argon plasma, and method using high frequency plasma is called a high frequency sputtering method, and method using direct-current plasma is called a direct-current sputtering method.
In general, a direct-current sputtering method has been utilized industrially in a wide range, because it provides higher film-formation rate and lower cost of power source facility and simpler film-formation operation, as compared with a high frequency sputtering method. However, a direct-current sputtering method has a disadvantage of requiring use of a conductive target, as compared with a high frequency sputtering method, which can provide film-formation even by using an insulating target.
Film-formation rate, in using a sputtering method for film-formation, has close relation to chemical bond of a target substance. Because a sputtering method utilizes a phenomenon that argon cations having a kinetic energy are collided to the target surface, and a substance of a target surface is flicked by receiving energy, weaker inter-ionic bond or inter-atomic bond of the target substance increases more probability jumping out by sputtering.
As a film-formation method of a transparent conductive film of an oxide such as ITO by using a sputtering method, there are a method for film-formation of an oxide film by a reactive sputtering method in mixed gas of argon and oxygen, by using an alloy target (an In—Sn alloy in the case of the ITO film) of elements constituting the film, and a method for film-formation of an oxide film by a reactive sputtering method in mixed gas of argon and oxygen, by using an oxide sintered body target (an In—Sn—O sintered body in the case of the ITO film) of elements constituting the film.
Among these, in a method for using the alloy target, relatively high amount of oxygen is supplied during sputtering, and because dependence of film-formation rate or film characteristics (specific resistance, transmittance) on amount of oxygen gas introduced during film-formation is extremely high, it is difficult to produce stably a transparent conductive film having a constant film thickness or desired characteristics. On the other hand, in a method using the oxide target, a part of oxygen to be supplied to a film is supplied from the target by sputtering, and residual deficient oxygen amount is supplied as oxygen gas. Therefore, dependence of film-formation rate or film characteristics (specific resistance, transmittance) on amount of oxygen gas introduced during film-formation is lower as compared with the case where the alloy target is used, and a transparent conductive film having a constant film thickness or characteristics can be produced more stably, and for this reason, a method for using the oxide target has been adopted industrially.
In consideration of productivity or production cost, a direct-current sputtering method is easier in high speed film-formation than a high frequency sputtering method. That is, in comparing film-formation rate by charging the same power to the same target, a direct-current sputtering method provides about 2 to 3 times higher rate. In addition, also in a direct-current sputtering method, charging of higher direct power is advantageous in view of productivity, because film-formation rate is more increased. Therefore, for industrial production, such a sputtering target becomes useful that is capable of providing film-formation stably, even when high direct-current power is charged.
On the other hand, an ion plating method is a film-formation method by locally heating the surface of a target material to be a film, by arc discharge, followed by sublimation, ionization and adhering onto a work charged minus. Both methods have features that a film with good adhesion can be provide at low temperature, extremely many kinds of substrate properties or film properties can be selected, film-formation of an alloy or a compound is possible, and is environmentally-friendly processes. An ion plating method, similarly to sputtering, is also capable of producing more stably a transparent conductive film having constant film thickness and characteristics, when an oxide tablet is used.
As described above, although an indium oxide-type material such as ITO has been used industrially in a wide range, in recent years, a non-indium-type material has been required, because indium, which is a rare metal, is expensive.
As the non-indium-type material, as described above, a zinc oxide-type material such as GZO or AZO, or a tin oxide-type material such as FTO or ATO is known. In particular, zinc oxide-type material has been noticed as a non-expensive material which is low cost, because of being underground abundantly as resource, and exhibits low specific resistance and high transmittance comparable to ITO. However, although a zinc oxide-type transparent conductive film obtained by using this has a merit of easy dissolution to usual acid and alkali, on the other hand, because it is poor in resistance to acid/alkali, and also difficult to control etching rate, it is difficult to be subjected to high precision patterning processing by wet etching, which is essential in a liquid crystal display application or the like. Therefore, applications thereof are limited to a solar cell and the like not requiring patterning. From these reasons, it has been a problem to improve chemical resistance of a zinc oxide-type material.
As an attempt to improve chemical resistance of a zinc oxide-type transparent conductive film, there are the following Examples. In Patent Document 1, there are proposals of a ZnO-type transparent conductive film by the co-addition of new impurities, a target material for producing the thin film and patterning technology, aiming at easy control of chemical characteristics of the ZnO-type transparent conductive film, without largely impairing visible light transmittance and electric specific resistance, by the co-addition of a donor impurity selected from aluminum (Al) or the like of group III elements or silicon (Si) or the like of group IV elements, together with chromium (Cr), to zinc oxide (ZnO).
However, Cr is known to have strong toxicity, therefore consideration should be paid so as not to give adverse effect on environment or a human body. In addition, because etching liquid must be controlled in a lower temperature range of from 20 to 5° C. than in a usual case, industrial application is difficult.
In addition, in Patent Document 2, there are proposals of an impurity co-added ZnO transparent conductive film in which cobalt (Co) or vanadium (V) is co-added instead of chromium (Cr), and a target material to be used for producing said thin film, or the like. However, cobalt (Co), similar to indium (In), is a rare metal. In addition, because vanadium (V) has toxicity, consideration should be paid so as not to give adverse effect on environment or a human body. In addition, in the addition of any of these, similarly to in Patent Document 1, because etching liquid must be controlled in a lower temperature range of from 20 to 5° C. than in a usual case, industrial application is difficult.
In addition, a method has been proposed for obtaining a transparent conductive film having improved crystallinity and etching characteristics and low resistance and excellent processability, without lowering mobility caused by scattering of ionized impurities (refer to Patent Document 3). Here, there is shown the formation (EXAMPLE 2) of an AZO film added with Mg, by using an RF magnetron sputtering method as a film-formation method, and by using a target affixed with an MgO chip onto a sintered body containing 2% by weight of ZnO:Al2O3. According to this method, it is described that the AZO film containing Mg in an atomic concentration of 5%, as ratio to Zn, in a film formed at a substrate temperature of 300° C., has a specific resistance reduced from 300 μΩcm to 200 μΩcm, and an etching rate by HCl enhanced 3 times, as compared with a film not containing Mg produced under the same condition. That is, it is shown that the addition of Mg rather lowers acid resistance, which is one of chemical resistance. In addition, there is no description on alkali resistance.
Under these circumstances, such a target has been required that does not contain components having toxicity giving adverse effect on environment or a human body, and is excellent in chemical resistance such as acid resistance, alkali resistance.    Patent Document 1: JP-A-2002-075061    Patent Document 2: JP-A-2002-075062    Patent Document 3: JP-A-8-199343